unix: have Makefile detect MIPS and x64 options

If no ARCH variable is passed, then the Makefile will attempt to detect platform-specific options (MIPS and non-x86-64.

Seems to work for native and cross-compilation (ARCH must be passed for the latter).
Maybe use `$or(...)` to combine this and the next ifeq.

Also, don't use override.  Instead append to `CFLAGS` (and then, for flow of Makefile, best to put this code after the "compiler settings" chunk below.